Looking for help with wiring a 1 field regulator to a 2 field alternator

I would not take the brushes apart to ground one. I would make CERTAIN (with a meter) the following:

With both field wires unhooked, take a meter and look for continuity between either of the field terminals and ground. It should be open/ infinity.

Next I would make up a short jumper from either field terminal and connect the other end to ground, leaving both field terminals "as they were."

WHY? Because this is what can confuse people and leads to burned wires. If at some point you forget, and thing maybe that is an isolated field (it is not "two field) you may connect a live wire to the grounded terminal by mistake

Second, if that old regulator fails, you can add one more wire and use a 70/ later regulator--which are better and which is what was used with the isolated field alternator.s